The smelting process is essentially very simple. Pellets of Galena are heated until they reach melting point. Because lead has a low melting point any impurities are not melted and are left as waste. The molten lead can be allowed to drain from the fire hearth into a collecting pot and then poured into moulds to form blocks of pure lead metal.
In practice things are not quite so simple:
- To achieve the temperatures necessary to melt the lead from the Galena requires a source of fuel. This was originally found by coppicing local woodland but was eventually supplied by cutting and drying moorland peat which was in plentiful supply.
- A constant stream of air, to aid combustion, was also needed and was provided initially by the use of hand bellows, but as mechanisation increased the use of a water wheel to drive huge bellows became common. This of course meant that water was needed and so mills were built in valleys to find a ready supply of water and then large scale earthworks produced a dam and reservoir to store and control the water.
- The heated Galena gives off poisonous fumes (particularly Sulfur Dioxide and also Vaporised Lead). In early small scale smelting these escaped into the atmosphere but in large scale mills the fumes needed to be removed efficiently. Large flues (horizontal chimneys) up to 1km in length were built up hillsides –usually topped with a vertical chimney.
Periodically boys were send into the flues to scrape condensed lead from the walls to return it for processing.